Our overall objective is to develop a well characterized genetic system in a eukaryotic cell line maintained in tissue culture. In order to achieve this goal, we have isolated multiply marked sublines of the stable aneuploid Chinese hamster ovary cell line, CHO. We now have lines which are auxotrophic with respect to proline and purine and which are also resistant to the folic acid antagonist, aminopterin. We are proceeding to analyze the precise biochemical nature of the purine lesions by biochemical and genetic techniques. In addition, we are utilizing inactivated Sendai virus mediated hybridization with the human cell line, Wi-38 to enable us to study the chromosomal location and linkage relationships of the markers.